At present, evolved 3rd generation radio access (Evolved Universal Terrestrial Radio Access, which will be referred to hereinbelow as “SUTRA”) and advanced 3rd generation radio access network (Evolved Universal Terrestrial Radio Access Network, which will be referred to hereinbelow as “EUTRAN”) are under investigation. These specifications are called long-term evolution (Long Term Evolution, which will be referred to hereinbelow as “LTE”). As the downlink of EUTRA, OFDMA (Orthogonal Frequency Division Multiplexing Access) schemes have been proposed (Non-Patent Document 1).
Further, as the next-generation EUTRA and EUTRAN, advanced long-term evolution (LTE-Advanced) has been proposed (Non-Patent Document 2). Also, as one of technologies applied to this, inter-cell cooperative reception has been proposed (Non-Patent Documents 3 and 4).
The contents of the above technologies will be briefly described hereinbelow.
1) Explanation on the Downlink Radio Frame Structure in EUTRA
As to the allocation of downlink radio channels in an OFDMA scheme, methods of multiplexing in time and frequency using resources spreading along the frequency axis (sub-carriers) of OFDM signals and the time axis (OFDM symbols), by time division multiplexing TDM (Time Division Multiplexing), frequency division multiplexing (Frequency Division Multiplexing) or combination of TDM and FDM have been proposed.
Further, in the technical specification document prepared by the international conference of 3GPP EUTRA technical investigation, a downlink radio frame configuration and a radio channel mapping method have been proposed.
FIG. 17 shows a configurational example of a downlink radio frame in EUTRA proposed by 3GPP, and is a diagram showing an example of radio channel mapping. The downlink radio frame shown in FIG. 17 is a group of multiple sub-carriers on the frequency axis (vertical axis) and is comprised of symbols arranged across a frequency bandwidth Bch and along the time axis (horizontal axis). As illustrated, one slot consists of 7 symbols, and two slots form one sub-frame. A two-dimensional radio resource block is formed of 12 sub-carriers by 7 symbols. Two consecutive radio resource blocks on the time axis forms a resource block pair (RG pair), which is enclosed by the bold line in FIG. 17. A plurality of resource block pairs (RB pairs) of this kind form one radio frame. Here, the minimum unit, formed by one sub-carrier and one OFDM symbol, is called a resource element.
For example, as shown in FIG. 17, the whole spectrum of the downlink (the system frequency bandwidth Bch specific to the base station) is 20 MHz wide on the frequency axis, one radio frame has a size of 10 ms, one subframe SF has a size of 1 ms, and 12 sub-carriers and one subframe (1 ms) form a resource block pair (RB pair). When the sub-carrier frequency bandwidth Bsc is assumed to be 15 kHz, the frequency bandwidth Bch of the resource block is 180 kHz (15 kHz×12). In downlink, 1200 sub-carriers are included in the whole 20 MHz band. One radio frame includes 100 RBs.
In the case of four transmission antennas shown in FIG. 17, it is understood that the reference signals RS0 of the first antenna (the reference signal in the figure is denoted such that RS0, for example is abbreviated to R0 by omitting S from RS) and the reference signals RS1 of the second antenna are included at the first, fifth, eighth and twelfth OFDM symbols. Similarly, the reference signals RS2 of the third antenna and the reference signals RS3 of the fourth antenna are distributed at the second and ninth OFDM symbols (see Non-Patent Document 1 below).
2) Explanation on Beam-Forming
Use of an array antenna having a plurality of antennas arranged therein makes it possible to realize a technology called the beam-forming stated above, whereby the gains and directivities of the beam patterns of the transmission/reception antennas are varied (Non-Patent Document 4). Application of beam-forming can enlarge the communication coverage distance and reduce interference in communication by multiple apparatuses. In LTE, when beam-forming is applied to transmission to a terminal unit, transmission is performed from the sixth antenna so that a reference signal RS5 specific to the terminal is transmitted from the sixth antenna. FIG. 18 shows one example of allocation of RS5. The ID code of reference signal RS5 is generated by performing an operation using an intra-cell terminal identifier (Cell-specific-Radio Network Temporary Identifier, which will be referred to hereinbelow as C-RNTI) that is allotted to every terminal and a cell identifier as variables, and is allocated upon communication with the terminal. The positions of allocation are shifted in the frequency direction, depending on the cell identifier.
3) Multi-User Multiple Input Multiple Output Scheme (MU-MIMO)
There has been a proposal of multiple input multiple output (Multi-Input Multi-Output, which will be referred to hereinbelow as MIMO) in which transmission and reception are carried out at the same frequency using a plurality of transmitting antennas and a plurality of receiving antennas so as to perform channel multiplexing by generating multiple channels between the transmitting and receiving antennas (see Non-Patent Document 5). MU-MIMO (multiple user MIMO) is a scheme in which MIMO is implemented using a plurality of terminals, and enables a plurality of terminals to perform communications simultaneously using the same frequency.
4) Explanation on Beam-Forming Using Direction of Arrival Estimation (Direction of Arrival, which Will be Referred to Hereinbelow as DOA)
To perform beam forming, there are two schemes: a closed-loop weight coefficient feedback scheme in which the terminal unit selects the weight coefficients for the individual transmitting antennas to be used to realize the directivity when the base station performs transmission to the terminal unit, so as to realize the optimal directivity, and transmits the weight coefficients to be used to the base station apparatus, then base station apparatus weights the transmitting antennas with the transmitted weight coefficients, to thereby realize the optimal directivity; and an open-loop weight coefficient scheme in which instead of transmission of weighting coefficients from the terminal unit, the base station selects weighting coefficients on its own right to perform weighting. As the open-loop weighting scheme, there is a proposal of a direction-of-arrival estimation (DOA) technique in which the base station apparatus estimates the direction of arrival from the uplink signal of the terminal unit and selects the weighing coefficients that realizes the directivity toward that direction to thereby perform transmission (Non-Patent Document 6).
5) Multiple Base Station Signal Cooperative Reception Scheme
In order to improve the reception characteristics of terminal units at the edge of cell, there are proposals of multiple base station signal cooperative reception schemes in which simultaneous transmissions are performed by a plurality of base stations so that the terminal unit can receive the signal from the multiple base stations (Non-Patent Documents 2 and 3). In these schemes, the signal from each of the multiple base station apparatuses are regarded as a transmission signal from one antenna so as to implement MIMO to obtain improvement of the reception quality thanks to transmission diversity effect or to thereby double the transmission capacity by virtue of spatial multiplexing effect. FIG. 19 shows the outline of a multiple base station signal cooperative reception scheme. Terminal unit UE100 not only receives a signal from base station apparatus 100, which is the main one, but also receives signals from surrounding base station apparatuses BS200, BS500, BS600 and BS700 at the same time. When there is a large distance between terminal unit UE100 and base station apparatus BS100, generally the reception characteristics lower. This degradation of the characteristics is suppressed by simultaneous reception of the signals from other base stations. Also, terminal unit UE101 also performs communication mainly with base station apparatus BS100. However, since the distance is short, a single base station BS300 is good enough as the extra base stations for implementation of cooperative reception. In this way, the positions and number of base stations to be used for cooperative reception are adaptively varied depending on the position of the terminal unit. The base stations are connected communication lines called back haul. For example, data to be transmitted to terminal unit UE101 is composed of data transmitted from base station apparatus BS100 and data transmitted from BS300 via the backhaul